LONDON — As Londoners reluctantly descended into the Underground subway system on Friday, the police announced the death toll from the rush-hour blasts had risen to more than 50.

Commission Sir Ian Blair said 100 victims from the bombings Thursday were hospitalized overnight, and 22 remain in critical condition.

Each explosive device was 10 pounds or less, Blair said.

Earlier, Britain's top law enforcement official warned that the bombers, whose attack bore the hallmarks of al-Qaida, could strike again.

"We have to have ... maximum consideration of the risk of another attack, and that's why our total effort today is focused on identifying the perpetrators and bringing them to justice," Home Secretary Charles Clarke told British Broadcasting Corp. radio early Friday morning.

"That is of course the No. 1 preoccupation that the police and security services have at this moment," he said.

Investigators said they would look for evidence in the debris from Thursday’s attacks and in the video footage from some 1,800 cameras in London’s train stations.

“There is real passion now in the police to make arrests quickly before further attacks can be carried out,” said Charles Shoebridge, a security analyst and former counterterrorism intelligence officer.

London’s mass transit system reopened Friday, though some commuters, admitting they were afraid, opted for a taxi. Others said they had little choice but to return to the Underground.

“I was scared, but what can you do?” said Raj Varatharaj, 32, emerging from a subway station. “This is the fastest way for me to get to work. You just have to carry on.”

Thursday’s blasts went off within 18 minutes at three subway stations, starting at 8:51 a.m. An explosion ripped the roof off a double-decker bus less than an hour later.

Hallmark of al-QaidaPrime Minister Tony Blair, who just the day before had been basking in glory of Britain’s successful Olympics bid, condemned the attacks and blamed Islamic extremists. Foreign Minister Jack Straw said the attacks
the attacks were in the style of al-Qaida
; the group responsible for Sept. 11.

Ten of London’s 12 subway lines reopened Friday, though service on three was restricted. Bus service was running through central London, except for diversions around blast sites.

Aldona Mosjko, a 21-year-old bagel shop manager from Poland, was among those too frightened to take public transportation Friday. “Normally, I take the bus, but today, I took a taxi. I was a bit afraid,” she said.

Detectives review footage, phone callsShoebridge said detectives will have to watch thousands of hours of video — slowly and carefully. Investigators will try to find on tape the point at which bombs were placed, then trace back the movements of the bomber, a task he said could involve hundreds of cameras.

Shoebridge said investigators also will check records of cell phone calls made in the bombed areas just before the explosions, a job that might be difficult if investigators can’t determine where bombers boarded the trains.

It was unclear whether there were any suicide bombers involved in the attacks. U.S. officials told NBC News that their British counterparts had discovered some timing devices associated with the attacks, though it was unclear where or how many.

Investigators doubt that cell phones — used in the Madrid train attacks a year ago — were used to detonate the bombs in the Underground because the phones often don’t work in the system’s tunnels, the official said. The official spoke on condition of anonymity because the investigation is ongoing.

'Potentially very credible' claim
The “Secret Group of al-Qaida’s Jihad in Europe” claimed it was behind the attacks, but the claim could not be immediately verified. In a posting on a Web site, the group said the bombings were punishment for Britain’s involvement in the war in Iraq and invasion of Afghanistan.

It threatened to attack Italy and Denmark for their support of the U.S.-led coalitions in both countries, too.

A senior U.S. counterterrorism official acknowledged that the Internet posting was considered a “potentially very credible” claim, in part because the message appeared soon after the attacks and didn’t appeared rushed. But no one was certain, and one defense official said it was too early to say.

The blasts paralyzed the city’s public transportation system Thursday, halting subway service, delaying buses and stranding thousands of residents and tourists.

The bombings happened as world leaders opened the G-8 summit in Scotland. The focus of this year’s summit was poverty reduction and climate change.

Frantic scenesScenes of frantic subway passengers covered in soot, some cut and bleeding and flooding out of subway stations flashed across television screens.

As explosions detonated in the vast subway that carries hundreds of thousands passengers a day, subway cars filled with smoke and many broke through the windows to escape or get air. Above ground, buses ferried the wounded and medics used a hotel as a makeshift emergency hospital.

“I didn’t hear anything, just a flash of light, people screaming, no thoughts of what it was, I just had to get out of the train,” said subway passenger Chris Randall, 28, who was hospitalized with cuts and burns on his face, legs and hands.

The worst attack on London since World War II brought out a stoicism that recalled Britain under the blitz of the Nazi Luftwaffe.

'Stoicism and resiliency'As Wednesday’s jubilation at winning the 2012 Summer Olympics gave way to the terrible shock of Thursday’s attacks, Blair rushed back to the capital and made a televised appeal for unity, praising the “stoicism and resiliency of the British people.”

Both were in evidence across the city, as volunteers helped the wounded from blast sites, commuters lent their phones so strangers could call home, and thousands faced long lines for homeward-bound buses or even longer walks without complaint.

“As Brits, we’ll carry on — it doesn’t scare us at all” said tour guide Michael Cahill, 37. “Look, loads of people are walking down the streets. It’s Great Britain — not called ’Great’ for nothing.”

“In the name of New York, Washington, Bali, Nairobi, Madrid and now London, we shall have vengeance and justice,” the Sun said in an editorial, reflecting on a worldwide conflict littered with many tombstones.

“We Britons will never be defeated,” said the Daily Express, which devoted 35 pages to the attacks.

Atop Buckingham Palace, the British flag flew at half-staff.

Security raised around the worldSecurity was raised in the United States and around the world. The Bush administration upped the terror alert a notch to code orange for the nation’s mass transit systems, and bomb-sniffing dogs and armed police patrolled subways and buses in the capital.

Much of Europe also went on alert, and Italy’s airports raised alert levels to a maximum.

Police said at least 700 were wounded. Among them, at least 45 were in serious or critical condition, including amputations, fractures and burns, hospital officials told The Associated Press.

At the scene of the bus explosion, bystander Raj Mattoo, 35, said the roof of the double-decker “flew off and went up about 10 meters (30 feet).”

Timeline of attacksDeputy Assistant Police Commissioner Brian Paddick said the first explosion hit a London Underground train that was 100 yards into a tunnel outside Moorgate station in the financial district in east London. Seven people died there, he said.

The second blast, at 8:56 a.m., was in the King's Cross station area of north London, and killed 21, Paddick said.

The third explosion, at 9:17 a.m., was near the Edgware station and killed seven people.

The bus explosion occurred at 9:47 a.m. At least two people were killed there.

Jay Kumar, a business owner near the site of the bus blast at Tavistock Square in central London, said he ran out of his shop when he heard a loud explosion. He said the bus’s top deck collapsed, sending people tumbling to the floor.

Many appeared badly injured, and bloodied people ran from the scene. “People were running this way panicked," he said. "They knew it was a bomb. Debris flying all over, mostly glass.”

Office worker Kibir Chibber said at one subway station that “I saw lots of people coming out covered in blood and soot. Black smoke was coming from the station. I saw several people laid out on sheets.”

Simon Corvett, on an eastbound train from Edgware Road station, described “this massive huge bang ... It was absolutely deafening and all the windows shattered.”

“You could see the carriage opposite was completely gutted,” he said. “There were some people in real trouble.”